Proficiency comes from repetition and nonjudgmental encouragement during practice. The phrase, "Good, can you do it better? Let's try again," combines praise with encouragement. There is nearly always something the child did well - tone, bow hold, position of the instrument - which can be praised.
Reviewing known pieces makes it possible for the student to reach a high level of excellence that deserves praise and in turn provides motivation to continue. When a child knows a piece and continues to work to refine it, words of praise are natural and the student relishes the well-deserved attention.